By the term “platinum group metal” (PGM) as used in this specification and claims is meant a metal selected from the group consisting of platinum, palladium, rhodium, osmium, ruthenium, iridium and rhenium.
Each of these PGMs is known to form volatile compounds of at least one of the three following groups.                1. A first group of PGM volatile compounds consists of volatile PGM halogen compounds, or complexes of PGM halogens with carbon monoxide. Some of the compounds from this group have a relatively high vapour pressure and relatively low decomposition temperature, which makes them suitable for a subsequent thermal decomposition process to produce the purified metal per se. Others are known to be difficult to synthesis or have extremely high temperatures of decomposition of the order of 600° C.        2. The second group of volatile PGM compounds are complexes of metal diketonates. All such PGM diketonates are solids with a high vapour pressure and most can be decomposed to deposit individual pure PGM metals. The metal diketonates are synthesized in solution, followed by solvent extraction. However, a difficulty with the use of these complexes for metal extraction and separation, is the unfavourable selectivity of the synthesis, as well as the similar sublimation temperatures of the diketonates.        3. The third group of volatile PGM compounds are PGM trifluorophosphine complexes. Most of the PGM trifluorophosphines are liquid and can be easily distilled at normal pressure without decomposition. The decomposition temperatures of PGM trifluorophosphines are, generally, between 140 and 340° C. The distinct exception is the trifluorophosphine complex of palladium which has a low thermal stability. Metals such as Ni, Co, Fe, Cr, Mo, Mn and W also form trifluorophosphine complexes. However, with the exception of Ni, Fe and Co, such complexes are solid and have very low vapor pressures [1].        
The main properties of metal trifluorophosphine complexes are presented in Table 1, while, general information about the synthesis of PGM trifluorophosphine complexes is presented in Tables 2, 3 and 4.
TABLE 1Physical properties of PGM of trifluorophosphines.MeltingBoilingDecompositionCompoundpointpointtemperatureHRh(PF3)4 [5]−40° C. 89° C.140° C.HIr(PF3)4 [5]−39° C. 95° C.245° C.HRe(PF3)5  42° C.Subl. 20° C.160° C./10 mmH2Os(PF3)4 [3]−72° C.280° C.340° C.H2Ru(PF3)4 [3]−76° C.180° C.290° C.Pt(PF3)4 [4]−15° C. 86° C.130° C.Pd(PF3)4 [4]−40° C.−20° C.
The general method of synthesis comprises the reduction of PGM salts with copper or zinc, under pressure, with phosphorus trifluoride. Phosphorus trifluoride (PF3) is a colorless gas with a boiling point of −101.8° C., has similar complexing properties to carbon monoxide and can be easily synthesized from phosphorus trichloride and zinc fluoride. There is no evidence about decomposition of PF3 during the thermal decomposition of the complexes; and the reagent gas can be recycled. This makes PF3 ideal for recycling as well as allowing of the deposition of ultra pure metals [6].
The trifluorophosphine complexes of PGM metals can be separated into two main groups, namely, trifluorophosphine metal hydrides and trifluorophosphine metals. The corresponding parameters for the synthesis of these two groups of compounds are represented in Tables 2 and 3. The trifluorophosphine metal hydrides are thermally and chemically stable. In aqueous systems, the complex hydrides HM(PF3)n are strong acids. Except in the case of HRh(PF3)4, the thermal release of hydrogen occurs only at high temperatures and with complete decomposition of the molecule. The decomposition process can be represented as follow:HM(PF3)n=nPF3+½H2+M
The resulting PF3 and H2 gas mixture can be recycled.
The thermal stability of volatile trifluorophosphine complexes of PGM is much lower than trifluorophosphine metal hydrides (Table 1). Palladium trifluorophosphine is stable only under a PF3 atmosphere. Platinum trifluorophosphine is decomposed at 130° C. The lower thermal stability of Pt and Pd complexes ma be used in their separation from other volatile trifluorophosphine complexes, especially the very volatile Ni(PF3)4. The thermal decomposition process can be represented as follows:M(PF3)n=nPF3+M
The resulting PF3 gas mixture may be recycled.
TABLE 2Parameters for the synthesis of thetrifluorophosphine complexes hydridesMXn + 4 PF3 + L½ H2 + nCu (Zn) =H1M(PF3)4 + n CuXStartingPressure (Bar)TemperaturematerialPF3H2° C.Yield %CoI2 [5]5030170100RhCl3 [5]9030170100IrCl3 [5]16045240100OsCl3 [3]40010027080RuCl3 [3]30010027070ReCl525010030040FeI2300100270traces
TABLE 3Parameters for the synthesis of the trifluorophosphine complexesMXn + 4 PF3 + nCu (Zn) = M(PF3)4 + n CuXStartingPressure PF3Temperaturematerial(Bar)° C.Yield %PtCl44010094PdCl230010080NiI2135100100FeI240018070
TABLE 4Parameters for the synthesis of thetrifluorophosphine complexes directlyfrom metals.StartingPressure PF3Temperaturematerial(Bar)° C.Yield %Pt [4]40100100Pd [4]25010095
Although, volatile individual PGM compounds and complexes as hereinbefore described are known to be formed and decomposed thermally to produce the pure metal, it is not known whether such processes are applicable when a plurality of PGMs are present together in varying degrees as various compounds, in such materials as, for example, ore, slag, scrap, slurry, concentrate, metallic intermediates, by-products and the like. This uncertainty is enhanced when other non-PGMs, such as, for example, Ni, Co, Fe, Cr, Mo. Mn and W are present and known to form complexes, such as, for example, with trifluorophosphine, and especially when some of these complexes, notably, Ni, Fe and Co are volatile with practical vapour pressures and thermally decomposable.
It is known, however, that PGMs do not always react with an aforesaid gaseous reactant to a sufficient extent in a satisfactory manner.
It is known that metals in the form of activated particulate metal are more reactive with reactant gases such as, for example, carbon monoxide and phosphorous trifluoride. The more “activated” the metal particulate, the more reactive and, thus, beneficial is the particulate in its reactivity with the aforesaid reactant gases. Of special value is the desire for enhanced activated particulate PGMs, for reaction with the aforesaid reactant gases selected from carbon monoxide, phosphorous trifluoride and mixtures thereof with hydrogen.
However, todate, the present PGM extraction processes suffer from being relatively expensive.
Accordingly, there is a need for an extraction and separation process adaptable to provide individual pure metals from various materials, comprising a plurality of such metals, in an efficacious, economic, and environmentally safe manner.